It is currently possible to capture/authorize more than an order's total price through our HPSDK. However, you wouldn't be able to change the price of the order itself, so I would recommend very strongly against doing this as it creates a negative user experience on the Shopify end of things.
It's also quite possible that this behaviour could change at any time, so if possible I would suggest keeping any logic to charge an additional fee separate to your own hosted page.
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